BRAIN CHIMERAS FOR THE STUDY OF AN AVIAN MODEL OF GENETIC EPILEPSY - STRUCTURES INVOLVED IN SOUND AND LIGHT-INDUCED SEIZURES

The epileptic homozygotes of the Fayoumi strain of chickens (Fepi) are affected by photogenic reflex epilepsy with complete penetrance. Here we demonstrate that they are equally affected by audiogenic reflex ep ilepsy induced by intense sound stimulation. All the Fepi display soun d-induced seizures from hatching to adulthood consisting of initial 'i ctal arousal' and running fits usually followed by generalized cronico -tonic convulsions. A running fit is the preconvulsive motor symptom s pecifically induced by auditory stimulation while neck myoclonus is th e preconvulsive motor symptom specifically induced by photic stimulati on. The EEG interictal spikes and spike and waves are suppressed and r eplaced by a desynchronized trace during the seizures of both kinds. V iable neural chimeras were obtained by graft of embryonic brain vesicl es from Fepi donors into normal chick embryos. Transfer of the complet e audiogenic and photogenic phenotypes was obtained in chimeras result ing from embryonic substitution of both the prosencephalon and mesence phalon. The substitution of the prosencephalon alone resulted in trans fer of interictal paroxysmal EEG activity accompanied by the sound and light-induced desynchronization and 'ictal arousal' with no motor sei zures. Chimeras with embryonic substitution of the mesencephalon alone displayed running fits and convulsions induced by sound stimulation b ut only neck myoclonus following light stimulation. The conclusions ar e reached that: (i) the Fepi is a model of audiogenic and photogenic r eflex epilepsy; (ii) in both types, the seizure initiator and the conv ulsion generator are localized in the brainstem, although reinforcemen t from telencephalic visual structures is needed to trigger photogenic generalized convulsions.